The principal components of dual-technology motion sensors comprise an ultrasonic and a passive infrared sensor. Ultrasonic sensors (also known as transceivers, when they both send and receive) work on a principle similar to radar or sonar which evaluates attributes of a target by interpreting the echoes from radio or sound waves. Ultrasonic sensors typically use a transducer, which generates sound waves in the ultrasonic range, above 20,000 hertz (20 kilohertz or 20 KHz), by turning electrical energy into sound, then upon receiving the echo turn the sound waves into electrical energy, which can be evaluated for evidence of motion in an area being monitored for control purposes. Changes in the phase, frequency (Doppler shift) or amplitude may be evaluated in the reflected echo. As the companion-sensor in a dual-technology motion sensor, a passive infrared (PIR) sensor is an electronic device that measures infrared (IR) light radiating from objects in its field of view. Apparent motion is detected when an infrared source with one temperature, such as a human, passes in front of an infrared source with another temperature, such as a wall. Infrared radiation enters through the front of the PIR sensor, its sensor face. At the core of a PIR sensor is a solid-state sensor or set of sensors, made from an approximately ¼ inch-square of natural or artificial pyroelectric materials. In a PIR-based motion detector, the PIR sensor is typically mounted on a printed circuit board containing the necessary electronics required to interpret the signals from the pyroelectric sensor chip. Infrared energy is able to reach the pyroelectric sensor typically through a window formed of material that is transparent to infrared radiation. The window may include focusing elements, such as a Fresnel lens or a mirror segment that may each be provided separately from the window. A person entering a room is detected when the infrared energy emitted from that person's body is focused by a Fresnel lens or a mirror segment and overlaps a section on the chip that had previously been looking at some much cooler part of the room. That portion of the chip is now much warmer than when the person wasn't there. As the person moves across the room, so does the hot spot on the chip's surface. This moving hot spot is evaluated by the electronics connected to the chip to perform a control function.
Containing both these types of internal sensors, a variety of motion sensors on the market today detect the presence of people in a room for the purpose of controlling a load such as automatically turning on/off lights or turning on/off electronic devices in a room. One such sensor-system uses an ultrasonic transmitter and receiver and a PIR sensor, such as the one disclosed in U.S. Pat. No. 5,189,393. Some dual technology or multiple technology sensors may use instead of or in addition to the PIR sensor other sensing technologies such as microwave, acoustic, vibration, imaging, electromagnetic, magnetic, and the like. One disadvantage of current ultrasonic systems is that ultrasonic sensors can interfere with other wireless devices.